Positive-electrode active material for non-aqueous secondary battery and method for producing the same

ABSTRACT

The present invention provides a positive-electrode active material for non-aqueous secondary battery comprising a sodium transition metal composite oxide represented by Formula: 
       Na x Fe 1-y M y O 2 ,         wherein 0.4≦x≦0.7, 0.25≦y&lt;1.0, and M is at least one element selected from the group consisting of manganese, cobalt and nickel, the sodium transition metal composite oxide having a crystal structure substantially composed of P6 3 /mmc alone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. 119 from Japanesepatent Application No. 2013-135703, filed on Jun. 28, 2013, Japanesepatent Application No. 2013-170718, filed on Aug. 20, 2013 and Japanesepatent Application No. 2014-075347, filed on Apr. 1, 2014, thedisclosures of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to positive-electrode active materials fornon-aqueous secondary batteries such as sodium ion secondary batteries,and to methods for producing the same.

2. Description of the Related Art

At present, non-aqueous secondary batteries, typically lithium ionsecondary batteries, have been in practical use in compact electronicdevices such as mobile phones and laptop computers. Lithium ionsecondary batteries have as high an operating voltage as about 4 V andcan store large amounts of energy per unit mass. Due to theseadvantages, their application to large apparatuses such as electricvehicles and power storage systems has been expected. In the lithium ionsecondary batteries, lithium transition metal composite oxides such aslithium cobaltate are typically used as the positive-electrode activematerials.

However, lithium and transition metals such as cobalt are rare orprecious elements due to reasons such as that the reserves of theseelements are unevenly distributed and also that such elements areobtained by separation as impurities from materials such as minerals.Under these circumstances, it has been proposed that sodium transitionmetal composite oxides having a layered structure based on abundantelements such as iron and sodium be used as the positive-electrodeactive materials. Sodium ion secondary batteries involving suchpositive-electrode active materials have been also proposed.

Patent Literature 1 discloses composite oxides represented byNaFe_(1-x)M_(x)O₂, wherein M is a trivalent metal element and 0≦x<0.5.The disclosure describes production methods involving Na₂O₂ and Fe₃O₄ asexamples of a sodium compound and an iron compound that are rawmaterials.

Patent Literature 2 discloses composite metal oxides with a layered rocksalt structure represented by Formula Na_(x)Fe_(1-y)M_(y)O₂, wherein Mis an element such as Mn, 0.5<x<1 and 0<y<0.5. According to thedisclosure, the above range of x ensures that the layered rock saltcrystal structure will have high purity and more sodium ions areavailable for doping and dedoping. The disclosure also describesproduction methods involving Na₂CO₃, Fe₃O₄ and MnO₂ as examples of asodium compound, an iron compound and a manganese compound that are rawmaterials.

Patent Literature 3 discloses composite metal oxides including a P2structure oxide and a layered oxide that are represented by FormulaNa_(x)Fe_(y)Mn_(1-y)O₂, wherein ⅔<x<1 and 0<y<⅔. The disclosuredescribes production methods involving Na₂CO₃, NaHCO₃, Na₂O₂, Fe₃O₄ andMnO₂ as examples of sodium compounds, an iron compound and a manganesecompound that are raw materials.

According to Non Patent Literature 1, the composite metal oxidesdescribed in Patent Literatures 2 and 3 have an R-3m crystal structurewhen the sodium to transition metal ratio is 1, but the layeredstructure comes to take a P6₃/mmc crystal structure when the ratio isless than 1. The P6₃/mmc structure is more resistant to breakage by thedesorption and insertion of sodium ions than the R-3m structure, andthis fact makes the P6₃/mmc crystal structure advantageous in terms ofcharge and discharge characteristics at a high stage of charge (SOC).

CITATION LIST

-   Patent Literature 1: JP 2005-317511A-   Patent Literature 2: JP 2009-135092A-   Patent Literature 3: WO 2012/060295-   Non Patent Literature 1: Nature Materials, Vol. 11, No. 6, pp.    512-517 (2012).

SUMMARY OF THE INVENTION

A first embodiment is a positive-electrode active material fornon-aqueous secondary battery comprising a sodium transition metalcomposite oxide that is represented by Formula Na_(x)Fe_(1-y)M_(y)O₂,wherein 0.4≦x≦0.7, 0.25≦y<1.0, and M is at least one element selectedfrom the group consisting of manganese, cobalt and nickel and having acrystal structure substantially composed of P6₃/mmc alone. Thepositive-electrode active material is satisfactory both incharge-discharge capacity and cycle characteristics and in otherproperties.

A second embodiment is a method for producing a positive-electrodeactive material for non-aqueous secondary battery comprising a sodiumtransition metal composite oxide represented by FormulaNa_(x)Fe_(1-y)M_(y)O₂, wherein 0.4≦x≦0.7, 0.25≦y<1.0, and M is at leastone element selected from the group consisting of manganese, cobalt andnickel, comprising a precipitate formation step of obtaining aprecipitate of a transition metal composite compound from a transitionmetal ion-containing aqueous solution, a heat treatment step of heattreating the precipitate from the precipitate formation step to obtain atransition metal composite oxide precursor, a mixing step of mixing theprecursor from the heat treatment step with at least a sodium compoundto obtain a raw material mixture, and a calcination step of calciningthe raw material mixture from the mixing step to obtain a calcinedproduct.

A third embodiment is a method for producing a positive-electrode activematerial for non-aqueous secondary battery comprising a sodiumtransition metal composite oxide represented by FormulaNa_(x)Fe_(1-y)M_(y)O₂, wherein 0.4≦x≦0.7, 0.25≦y<1.0, and M is at leastone element selected from the group consisting of manganese, cobalt andnickel, comprising a precipitate formation step of obtaining aprecipitate of a transition metal composite compound other than ahydroxide from a transition metal ion-containing aqueous solution, amixing step of mixing the precipitate from the precipitate formationstep with at least a sodium compound to obtain a raw material mixture,and a calcination step of calcining the raw material mixture from themixing step to obtain a calcined product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of XRD spectra showing data of positive-electrodeactive materials of the embodiment and comparative positive-electrodeactive materials.

FIG. 2 illustrates an example of moisture absorption data ofpositive-electrode active materials of the embodiment and comparativepositive-electrode active materials.

FIG. 3 illustrates an example of cycle characteristics of non-aqueoussecondary batteries involving positive-electrode active materials of theembodiment and comparative positive-electrode active materials in thepositive-electrodes.

DETAILED DESCRIPTION OF THE INVENTION

Although Patent Literatures 2 and 3 mention that a mixture ofmetal-containing compounds with a prescribed chemical composition may beobtained by a crystallization method, the specifications do not teachany specific conditions in the production methods or whatsoever. Themethods disclosed in Patent Literatures 1 to 3 involve the calcinationof a raw material mixture produced by a so-called dry process. However,sodium transition metal composite oxides obtained by such methodscontain subphases which have crystal structures different from thedesired structure. When the target crystal structure is P6₃/mmc,subphases such as R-3m structure, Pnma structure and Fd-3m structureoccur. The occurrence of such subphases is probably ascribed to theelements in the raw material mixture being mixed with insufficientuniformity.

On the other hand, the mixing of transition metal elements may beeffected to sufficient uniformity by a so-called coprecipitation processin which transition metal ions are precipitated by pH control or with anagent such as a complexing agent. However, counter ions for thetransition metal ions, for example, nitrate ions in the case of anaqueous transition metal nitrate salt solution, are often incorporatedinto the crystal or the precipitate aggregate during precipitation. Suchcounter ions that have been incorporated can adversely affect theperformance of non-aqueous secondary batteries. Thus, the bestperformance of sodium ion secondary batteries cannot be achieved by thesimple application of a coprecipitation process.

The present invention has been made in view of these circumstances. Anobject of the invention is to provide methods for producingpositive-electrode active materials for non-aqueous secondary batterywhich have a high purity of P6₃/mmc structure and allow non-aqueoussodium secondary batteries such as sodium ion secondary batteries tofully exhibit their performance. Another object of the invention is toprovide positive-electrode active materials for non-aqueous sodiumsecondary batteries which are satisfactory both in charge-dischargecapacity and cycle characteristics and in other properties.

The present inventor carried out extensive studies in order to achievethe above objects, completing the present invention. The presentinventor has found that a sodium transition metal composite oxide whichis substantially composed of P6₃/mmc alone and has a very low content ofcounter ions may be obtained by a process in which a transition metalcomposite compound as a precursor is prepared by a coprecipitationmethod including specific steps, the precursor is then mixed with otherraw material compounds, and the mixture is calcined. Specifically, thescope of the present invention includes the following aspects.

A first aspect of a method for producing a positive-electrode activematerial for non-aqueous secondary battery of the invention resides in amethod for producing, the positive-electrode active material fornon-aqueous secondary battery including a sodium transition metalcomposite oxide represented by Formula Na_(x)Fe_(1-y)M_(y)O₂, wherein0.4≦x≦0.7, 0.25≦y<1.0, and M is at least one element selected from thegroup consisting of manganese, cobalt and nickel, the method comprisinga precipitate formation step of obtaining a precipitate of a transitionmetal composite compound from a transition metal ion-containing aqueoussolution, a heat treatment step of heat treating the precipitate fromthe precipitate formation step to obtain a transition metal compositeoxide precursor, a mixing step of mixing the precursor from the heattreatment step with at least a sodium compound to obtain a raw materialmixture, and a calcination step of calcining the raw material mixturefrom the mixing step to obtain a calcined product.

A second aspect of a method for producing a positive-electrode activematerial for non-aqueous secondary battery of the invention resides in amethod for producing the positive-electrode active material fornon-aqueous secondary battery including a sodium transition metalcomposite oxide represented by Formula Na_(x)Fe_(1-y)M_(y)O₂, wherein0.4≦x≦0.7, 0.25≦y<1.0, and M is at least one element selected from thegroup consisting of manganese, cobalt and nickel, the method comprisinga precipitate formation step of obtaining a precipitate of a transitionmetal composite compound other than a hydroxide from a transition metalion-containing aqueous solution, a mixing step of mixing the precipitatefrom the precipitate formation step with at least a sodium compound toobtain a raw material mixture, and a calcination step of calcining theraw material mixture from the mixing step to obtain a calcined product.

A positive-electrode active material for non-aqueous secondary batteryof the invention includes a sodium transition metal composite oxide thatis represented by Formula Na_(x)Fe_(1-y)M_(y)O₂, wherein 0.4≦x≦0.7,0.25≦y<1.0, and M is at least one element selected from the groupconsisting of manganese, cobalt and nickel and has a crystal structuresubstantially composed of P6₃/mmc alone.

By virtue of the configurations mentioned above, the inventive methodsfor the production of positive-electrode active materials fornon-aqueous secondary battery can produce sodium transition metalcomposite oxides which are substantially composed of a P6₃/mmc structurealone and have a sufficiently low content of counter ions contained inthe oxide.

By virtue of the configurations mentioned above, the inventivepositive-electrode active material for non-aqueous secondary batteryallows a battery to achieve advantageous charge-discharge capacity andcycle characteristics due to the P6₃/mmc structure and also to performwell in other battery characteristics.

In the present specification, the term “step” as used herein encompassesnot only an independent step but also a step in which the anticipatedeffect of this step is achieved, even if the step cannot be clearlydistinguished from another step. Unless specifically indicated, in acase in which each ingredient of a composition includes pluralmaterials, the content of each ingredient of the composition denotes thetotal amount of the plural materials included in the composition.

Hereinbelow, non-aqueous secondary battery positive-electrode activematerials and methods for producing the active materials according tothe present invention will be described in detail with reference toembodiments and examples.

1. Positive-Electrode Active Materials for Non-Aqueous Secondary Battery

First, the positive-electrode active materials for non-aqueous secondarybattery, hereinafter, also written simply as “positive-electrode activematerials”, according to the present invention will be discussed indetail.

The positive-electrode active material for non-aqueous secondary batteryof the invention includes a sodium transition metal composite oxide thatis represented by Formula Na_(x)Fe_(1-y)M_(y)O₂, wherein 0.4≦x≦0.7,0.25≦y<1.0, and M is at least one element selected from the groupconsisting of manganese, cobalt and nickel and has a crystal structuresubstantially composed of P6₃/mmc alone.

Here, the presence of a P6₃/mmc structure in the crystal structure maybe identified based on, for example, a powder X-ray diffractometry (XRD)spectrum. The phrase “substantially composed of P6₃/mmc alone” meansthat the crystal structure may include other subphases in addition tothe P6₃/mmc structure as long as the advantageous effects of theinvention are achieved. Specifically, the content of the P6₃/mmcstructure is not less than 95%, and preferably not less than 98% of thecrystal structure.

1-1. Chemical Composition

The chemical composition of the sodium transition metal composite oxidethat is the main component is represented by the above formula. Bylimiting x to 0.4≦x≦0.7, the main crystal structure of the sodiumtransition metal composite oxide is P6₃/mmc and the breakage of thecrystal structure by the desorption and insertion of sodium ions isprevented. It should be noted that the crystal structure starts to shiftfrom the P6₃/mmc if the value of x is outside the above range. Forexample, the crystal takes an R-3m structure when x is around 1. Apreferred range is 0.5≦x≦0.7. While conventional sodium transition metalcomposite oxides with a P6₃/mmc structure inevitably contain subphaseshaving other crystal structures, the sodium transition metal compositeoxide obtained by the producing methods of the invention issubstantially composed of a P6₃/mmc structure alone.

By limiting y to 0.25≦y<1.0, the obtainable P6₃/mmc sodium transitionmetal composite oxide attains good crystallinity. It should be notedthat any value of y outside this range causes a decrease incrystallinity and/or the occurrence of subphases. A preferred range is0.25≦y≦0.75.

The letter M is at least one element selected from the group consistingof manganese, cobalt and nickel having a similar ion radius to iron. Theuse of these elements M advantageously makes it easy to obtain thedesired crystal structure. When M is manganese, the target material witha stable crystal structure may be obtained easily.

1-2. Oxo Acid Ions

The positive-electrode active material of the invention sometimescontains a trace amount of oxo acid ions. According to ICP-AES(inductively coupled plasma atomic emission spectrometry), the contentof oxo acid ions is preferably not more than 0.3 wt %, more preferablynot more than 0.1 wt %, further preferably not more than 0.05 wt %, andparticularly preferably below the detection limit or around thedetection limit (about 300 ppm). Examples of the oxo acid ions includesulfate ions and nitrate ions.

1-3. Crystal Structures

Preferably, the crystal structure of the positive-electrode activematerial of the invention shows a specific characteristic in powderX-ray diffractometry. When, for example, the peak intensity assigned tothe (110) plane has a high ratio to the peak intensity assigned to the(016) plane, hereinafter, also written as “(110)/(016)”, the crystalstructure achieves higher strength and becomes more resistant tobreakage by the desorption and insertion of sodium ions, resulting inimproved cycle characteristics. The peak intensity ratio is preferablynot less than 0.30. A peak intensity ratio higher than 2.00 indicates apossibility of the presence of crystal phases other than the P6₃/mmcphase. Thus, the peak intensity ratio is preferably not more than 2.00,more preferably 0.35 to 1.00, and particularly preferably 0.40 to 0.80.

The (110) plane shows a diffraction peak in the range where 2θ is 61.2°to 61.7°, and the (016) plane shows a diffraction peak in the rangewhere 2θ is 63.3° to 63.8°.

The integral widths of the (016) plane peak and the (110) plane peakserve as an indicator of how well the sodium transition metal compositeoxide has been crystallized. Smaller widths are more preferable. Theintegral width is preferably not more than 1.00°, and more preferablynot more than 0.50° for the (016) plane peak, and is preferably not morethan 0.30°, and more preferably not more than 0.25° for the (110) planepeak. A realistic value of the integral width is 0.10° or above for bothpeaks.

In the positive-electrode active material, the content of the sodiumtransition metal composite oxide represented by the above formula is notparticularly limited. For example, the content may be 80 mass % or more,and preferably 95 mass % or more. It is more preferable that thepositive-electrode active material is substantially composed of thesodium transition metal composite oxide represented by the above formulaalone. The term “substantially” means that the positive-electrode activematerial may include compounds other than the sodium transition metalcomposite oxide represented by the above formula as long as theadvantageous effects of the invention are achieved.

The positive-electrode active materials for non-aqueous secondarybattery of the invention may be preferably produced by any of thefollowing production methods which advantageously allow for efficientproduction.

2. Methods for Producing Positive-Electrode Active Materials forNon-Aqueous Secondary Battery

Next, there will be described the methods for producingpositive-electrode active materials for non-aqueous secondary batteryaccording to the invention. The inventive methods for producingpositive-electrode active materials for non-aqueous electrolytesecondary battery may be performed largely in two embodiments.

2-1. First Embodiment

In the method for producing a positive-electrode active material fornon-aqueous secondary battery of the invention, the first embodiment isa method for producing a positive-electrode active material fornon-aqueous secondary battery including a sodium transition metalcomposite oxide represented by Formula Na_(x)Fe_(1-y)M_(y)O₂, wherein0.4≦x≦0.7, 0.25≦y<1.0, and M is at least one element selected from thegroup consisting of manganese, cobalt and nickel, and is characterizedby including a precipitate formation step of obtaining a precipitate ofa transition metal composite compound from a transition metalion-containing aqueous solution, a heat treatment step of heat treatingthe precipitate from the precipitate formation step to obtain atransition metal composite oxide precursor, a mixing step of mixing theprecursor from the heat treatment step with at least a sodium compoundto obtain a raw material mixture, and a calcination step of calciningthe raw material mixture from the mixing step to obtain a calcinedproduct.

2-1-1. Chemical Composition

The sodium transition metal composite oxide that is the main componentin the target positive-electrode active material is represented by theabove formula. Details are as mentioned in the description of thepositive-electrode active materials for non-aqueous secondary batteryaccording to the invention.

2-1-2. Precipitate Formation Step

In the precipitate formation step, a precipitate of a transition metalcomposite compound is obtained from a transition metal ion-containingaqueous solution. The precipitate formation step is preferably a step inwhich a basic compound such as sodium hydroxide is added to thetransition metal ion-containing aqueous solution to adjust the pH and toobtain a precipitate of a poorly soluble transition metal compositecompound.

The transition metal ion-containing aqueous solution may beappropriately prepared by, for example, dissolving transition metalcompounds such as chlorides, sulfate salts and nitrate salts into anacid or pure water, or by dissolving transition metals into an acid. Anyappropriate acids such as hydrochloric acid, nitric acid and sulfuricacid may be selected in accordance with the solutes. In view of factorssuch as loads to the facility and the environment, availability andeasiness in handling, the transition metal ion-containing aqueoussolution that is obtained is preferably an aqueous sulfate saltsolution. When an aqueous sulfate salt solution is used, the heattreatment step described later has a particular importance.

The transition metal ion-containing aqueous solution contains at leastiron ions, and further contains ions of at least one transition metalselected from the group consisting of manganese, cobalt and nickel(hereinafter, also written as specific transition metal ions). The ratioof the content of iron ions to the content of specific transition metalions may be selected appropriately in accordance with the chemicalcomposition of the target transition metal composite compound.

Examples of the poorly soluble transition metal composite compoundsinclude hydroxides, carbonate salts and oxalate salts. From theviewpoint of handling, hydroxides are preferable.

2-1-3. Heat Treatment Step

In the heat treatment step, the transition metal composite compoundobtained in the precipitation step is heat treated to form a transitionmetal composite oxide precursor. This step removes the counter ions (forexample, sulfate ions when the transition metal ion-containing aqueoussolution is an aqueous sulfate salt solution) from the transition metalcomposite compound, resulting in a transition metal composite oxideprecursor which contains less impurities and has been crystallized to adegree. The counter ions remaining in the precursor cause a decrease inthe crystallinity of the final sodium transition metal composite oxide.When, in particular, the counter ions are sulfate ions, the importanceof this step further increases because the sulfate ions will remain evenin the final sodium transition metal composite oxide to possiblyadversely affect moisture absorption properties and various batterycharacteristics.

The temperature of the heat treatment requires careful control becausethe treatment at an excessively low temperature results in insufficientcrystallization and insufficient counter ion removal while too hightemperatures cause sintering to proceed excessively and undesired phasesto occur. To fulfill the purpose, the heat treatment temperature ispreferably in the range of 600° C. to 1000° C., although the tendenciesof heat treatment vary slightly depending on the chemical composition.The heating temperature is more preferably 800° C. to 950° C. becausethe unity or uniformity of the crystal structure is markedly increased.

The heat treatment is performed for at least a certain time periodbecause too short a treatment time does not allow the reaction tocomplete. The heat treatment time may be extended to any extent withouta problem. However, performing the heat treatment for an overly longtime only protracts the step and is thus not necessary. In view ofthese, the heat treatment time is preferably 0.5 hours to 50 hours, andmore preferably 3 hours to 24 hours.

The heat treatment step may be performed in any atmosphere withoutlimitation, but is preferably carried out in an oxidizing atmosphere.Examples of the oxidizing atmospheres include air atmosphere andoxygen-containing atmospheres.

2-1-4. Mixing Step

In the mixing step, the precursor is mixed with at least a sodiumcompound to give a raw material mixture. The sodium compounds may be anycompounds which can be decomposed into oxides at high temperatures, withexamples including sodium carbonate, sodium hydroxide, sodium oxide,sodium peroxide, sodium chloride, sodium nitrate and sodium sulfate. Inparticular, sodium oxide and sodium peroxide are preferable due to theirhigh reactivity in the calcination step, but this characteristic alsorequires careful attention in the mixing step such as the need ofhandling the compound in an inert atmosphere such as nitrogen or argon.Sodium carbonate is advantageously easy to handle in the mixing step.The sodium compounds such as sodium nitrate should be handledsufficiently carefully although the counter ions in these compounds areremoved more easily in the subsequent calcination step compared to thecounter ions contained in the precursor. From these viewpoints, sodiumcompounds other than strong acid salts are preferable such as sodiumcarbonate, sodium hydroxide, sodium oxide and sodium peroxide.

The mixing ratio of the precursor to the sodium compound in the rawmaterial mixture may be selected appropriately in accordance with thechemical composition of the target sodium transition metal compositeoxide. The raw material mixture may further contain additives such assintering auxiliaries (fluxes) in accordance with the purpose.

The mixing step may involve using any of known mixers such as ballmills, twin-cylinder mixers and stirrers.

2-1-5. Calcination Step

In the calcination step, the raw material mixture from the mixing stepis calcined to give a calcined product. Any of known calcination meansmay be selected appropriately in accordance with the purpose. Forexample, the raw material mixture may be compacted before thecalcination or may be charged into a crucible directly.

Examples of the calcination furnaces which may be used include batchfurnaces, tunnel furnaces and rotary kilns. The calcination temperatureneeds to be controlled carefully because too low calcinationtemperatures cause undesired subphases to occur and too high calcinationtemperatures cause sintering to proceed excessively. Calcinationtemperatures of 700° C. to 1100° C. advantageously ensure that acalcined product with a P6₃/mmc phase may be obtained. The calcinationtemperature is more preferably 800° C. to 1000° C.

The calcination step may be performed in any atmosphere withoutlimitation, but is preferably carried out in an oxidizing atmosphere.Examples of the oxidizing atmospheres include air atmosphere andoxygen-containing atmospheres.

2-1-6. Additional Steps

The calcined product obtained above may be subjected to treatments suchas pulverization, washing and sieving as required, thereby obtaining apositive-electrode active material for non-aqueous secondary batteryincluding the target sodium transition metal composite oxide.

2-2. Second Embodiment

In the method for producing a positive-electrode active material fornon-aqueous secondary battery of the invention, the second embodiment isa method for producing a positive-electrode active material fornon-aqueous secondary battery including a sodium transition metalcomposite oxide represented by Formula Na_(x)Fe_(1-y)M_(y)O₂ (wherein0.4≦x≦0.7, 0.25≦y<1.0, and M is at least one element selected from thegroup consisting of manganese, cobalt and nickel), and is characterizedby including a precipitate formation step of obtaining a precipitatebased on (namely, containing as a main component) a transition metalcomposite compound other than a hydroxide from a transition metalion-containing aqueous solution, a mixing step of mixing the precipitatefrom the precipitate formation step with at least a sodium compound toobtain a raw material mixture, and a calcination step of calcining theraw material mixture from the mixing step to obtain a calcined product.

2-2-1. Chemical Composition

The chemical composition is similar to that described in the firstembodiment.

2-2-2. Precipitate Formation Step

In the precipitate formation step, a precipitate based on a transitionmetal composite compound other than a hydroxide is obtained from atransition metal ion-containing aqueous solution. The precipitateformation step is preferably a step in which a basic compound such assodium hydroxide is added to the transition metal ion-containing aqueoussolution to adjust the pH, and a specific precipitating agent is addedto obtain a precipitate of a poorly soluble transition metal compositecompound. This precipitate is based on a transition metal compositecompound derived from the precipitating agent, and contains no or littlehydroxide. Although variable depending on conditions such as the pHvalue adjusted and the precipitating agent used, the content ofhydroxide ions in the precipitate is preferably about 3 mol % or less,and particularly preferably 1 mol % or less relative to 100 mol % of theanions derived from the precipitating agent.

The transition metal ion-containing aqueous solution may beappropriately prepared by, for example, dissolving transition metalcompounds such as chlorides, sulfate salts and nitrate salts into anacid or pure water, or by dissolving transition metals into an acid. Anyappropriate acids such as hydrochloric acid, nitric acid and sulfuricacid may be selected in accordance with the solutes. In view of factorssuch as loads to the facility and the environment, availability andeasiness in handling, the transition metal ion-containing aqueoussolution that is obtained is preferably an aqueous sulfate saltsolution.

The transition metal ion-containing aqueous solution contains at leastiron ions, and further contains at least one type of specific transitionmetal ions selected from the group consisting of manganese, cobalt andnickel. The ratio of the content of iron ions to the content of specifictransition metal ions may be selected appropriately in accordance withthe chemical composition of the target transition metal compositecompound.

Examples of the poorly soluble transition metal composite compoundsinclude carbonate salts and oxalate salts. Carbonate salts arepreferable from the viewpoint of the relation with the precipitatingagent as will be described below and also from the viewpoint of theperformance of the positive-electrode active material.

A precipitate obtained by pH adjustment alone is based on a hydroxide,hereinafter, also written as hydroxide precipitate for convenience. Ahydroxide precipitate formed sometimes contains the counter ions presentin the transition metal ion-containing aqueous solution, for example,sulfate ions in the case of an aqueous sulfate salt solution. Theremaining of the counter ions can cause a decrease in the crystallinityof the final sodium transition metal composite oxide. When, inparticular, the counter ions are sulfate ions, the sulfate ions willremain even in the final sodium transition metal composite oxide topossibly adversely affect moisture absorption properties and variousbattery characteristics. To prevent such problems, a precipitating agentis used in combination with the pH adjustment to form a precipitate thatis based on a transition metal composite compound other than ahydroxide. In this manner, the precipitate is prevented from containingan excessively large amount of undesired counter ions.

Examples of the precipitating agents include carbon dioxide,water-soluble carbonate salts, oxalic acid and water-soluble oxalatesalts. From viewpoints such as easiness in handling and costs, carbondioxide and water-soluble carbonate salts are preferable, and carbondioxide is particularly preferable.

2-2-3. Heat Treatment Step

Prior to the mixing step described below, the transition metal compositecompound from the precipitate formation step may be subjected to a heattreatment to form a transition metal composite oxide. Even in the casewhere a slight amount of a hydroxide precipitate has been formed in theprecipitate formation step, this heat treatment removes sufficiently thecounter ions from the precipitate of the transition metal compositecompound. Although variable depending on the chemical composition, theheat treatment temperature may be appropriately 600° C. to 1000° C. Atsuch temperatures, the counter ions may be removed to a sufficientextent without causing excessive sintering. For details in the heattreatment step, refer to the description of the heat treatment step inthe first embodiment.

2-2-4. Mixing Step

In the mixing step, the precipitate, or the transition metal compositeoxide from the heat treatment step, is mixed with at least a sodiumcompound to give a raw material mixture. The sodium compounds may be anycompounds which can be decomposed into oxides at high temperatures, withexamples including sodium carbonate, sodium hydroxide, sodium oxide,sodium peroxide, sodium chloride, sodium nitrate and sodium sulfate. Inparticular, sodium oxide and sodium peroxide are preferable due to theirhigh reactivity in the calcination step, but this characteristic alsorequires careful attention in the mixing step such as the need ofhandling the compound in an inert atmosphere such as nitrogen or argon.Sodium carbonate is advantageously easy to handle in the mixing step.The sodium compounds such as sodium nitrate should be handledsufficiently carefully although the counter ions in these compounds areremoved more easily in the subsequent calcination step compared to thecounter ions contained in the precipitate. From these viewpoints, sodiumcompounds other than strong acid salts are preferable such as sodiumcarbonate, sodium hydroxide, sodium oxide and sodium peroxide.

The mixing ratio of the precipitate to the sodium compound in the rawmaterial mixture may be selected appropriately in accordance with thechemical composition of the target sodium transition metal compositeoxide. The raw material mixture may further contain additives such assintering auxiliaries (fluxes) in accordance with the purpose.

The mixing step may involve using any of known mixers such as ballmills, twin-cylinder mixers and stirrers.

2-2-5. Calcination Step

The calcination step is performed in accordance with the firstembodiment.

2-2-6. Additional Steps

Additional steps may be performed in accordance with the firstembodiment.

3. Non-Aqueous Secondary Batteries

Next, an example will be described in which non-aqueous secondarybatteries are manufactured with the positive-electrode active materialsfor non-aqueous secondary battery of the invention.

3-1. Positive-Electrodes

The positive-electrode active material for non-aqueous secondary batterymay be mixed with known components such as a conductive material and abinder to give a positive-electrode mixture, which is then applied to aknown positive-electrode collector to form a positive-electrode activematerial layer. Thus, a positive-electrode for non-aqueous secondarybattery may be obtained.

Examples of the conductive materials include natural graphite,artificial graphite and acetylene black. Examples of the binders includepolyvinylidene fluoride, polytetrafluoroethylene and polyamide acrylicresin.

Examples of the materials of positive-electrode collectors includealuminum, nickel and stainless steel.

For example, the positive-electrode active material layer may be formedby dispersing the positive-electrode mixture in a solvent, applying thedispersion to the positive-electrode collector, and drying and pressingthe wet coating, or may be formed by directly providing thepositive-electrode mixture on the positive-electrode collector bypressure forming

3-2. Non-Aqueous Secondary Batteries

A non-aqueous secondary battery may be obtained using thepositive-electrode for non-aqueous secondary battery obtained above andother known components such as a negative electrode for non-aqueoussecondary battery, a non-aqueous electrolytic solution or a solidelectrolyte, and a separator.

The negative electrode for non-aqueous secondary battery may be obtainedby applying a known negative electrode active material for non-aqueoussecondary battery on a known negative electrode collector to form anegative electrode active material layer.

Examples of the negative electrode active materials include metallicsodium, sodium alloys and materials capable of doping and dedoping ofsodium ions. Exemplary materials capable of doping and dedoping ofsodium ions include carbonaceous materials, chalcogen compounds such asoxides and sulfides capable of sodium ion doping and dedoping at lowerpotential than the positive-electrode, and borate salts. Wherenecessary, the negative electrode active material may form a negativeelectrode mixture with a thermoplastic resin as a binder. Examples ofthe thermoplastic resins include polyvinylidene fluoride, polyethyleneand polypropylene.

Examples of materials for the negative electrode collectors includealuminum, copper, nickel and stainless steel. Aluminum is preferable dueto easiness in thin film production and inexpensiveness. In contrast tolithium ion batteries, sodium ion batteries allow use of aluminum asnegative electrode collectors. Thus, sodium ion batteries surpasslithium ion batteries in terms of cost and easy handling. For example,the negative electrode active material layer may be formed by directlyproviding the negative electrode active material or the negativeelectrode mixture on the negative electrode collector by pressureforming, or may be formed by dispersing the negative electrode activematerial optionally together with other components in a solvent,applying the dispersion on the negative electrode collector, and dryingand pressing the wet coating.

When the transportation of sodium ions is mediated by a non-aqueouselectrolytic solution, the solvent of the electrolytic solution may bean organic solvent. Examples thereof include dimethoxyethane,diethoxyethane, ethylene carbonate, propylene carbonate, dimethylcarbonate, diethyl carbonate, methyl ethyl carbonate, methyl formate,γ-butyrolactone, 2-methyltetrahydrofuran, dimethylsulfoxide andsulfolane.

Examples of the electrolytes include sodium salts such as sodiumperchlorate, sodium tetrafluoroborate, sodium hexafluorophosphate andsodium trifluoromethanesulfonate. The electrolytic solution may begelled by the addition of an agent such as a gelling agent. Further, theelectrolytic solution may be absorbed into an absorbent polymer. Thesodium ion concentration in the electrolytic solution may be adjustedappropriately in accordance with the purpose.

When sodium ions are transported via a solid electrolyte, for example, apolymer compound having a polyethylene oxide backbone, or a polymercompound containing at least one of polyorganosiloxane chains andpolyoxyalkylene chains may be used. Further, solid electrolytesincluding inorganic compounds may be used.

Examples of the separators include porous films such as of polyethyleneand polypropylene.

The shapes of the non-aqueous secondary batteries are not particularlylimited and may be determined in accordance with any knownconfigurations and appropriate modifications of such known shapes.

EXAMPLES

Hereinbelow, the present invention will be described in further detailbased on Examples without limiting the scope of the invention.

First, the first embodiment will be described based on some examples.

Example 1 Precipitate Formation Step

Iron (II) sulfate and manganese sulfate were dissolved in pure watersuch that the ratio of the amounts thereof by mole would be Fe:Mn=1:1.Thus, a reaction field consisting of an aqueous transition metal sulfatesalt solution was prepared. The liquid temperature of the reaction fieldwas adjusted to 50° C. While performing stirring, a prescribed amount ofan aqueous sodium hydroxide solution was dropped to the reaction field,thereby obtaining a precipitate of a transition metal compositehydroxide represented by the composition formula(Fe_(0.5)Mn_(0.5))(OH)₂.

Heat Treatment Step

The precipitate was separated from the reaction field and was heattreated in air at 900° C. for 12 hours to give a transition metalcomposite oxide precursor represented by the composition formula(Fe_(0.5)Mn_(0.5))₂O₃, hereinafter, also written as “transition metalprecursor”.

Mixing Step

After the heat treatment, 0.35 mol of sodium carbonate and 1.0 mol ofthe transition metal precursor were mixed with each other to give a rawmaterial mixture.

Calcination Step

The raw material mixture was calcined in air at 900° C. to give apositive-electrode active material represented by the compositionformula Na_(0.7)(Fe_(0.5)Mn_(0.5))O₂.

Example 2

A positive-electrode active material represented by the compositionformula Na_(0.7)(Fe_(0.5)Mn_(0.5))O₂ was obtained in the same manner asin Example 1, except that the heat treatment temperature in the heattreatment step was changed to 650° C.

Comparative Example 1

A positive-electrode active material represented by the compositionformula Na_(0.7)(Fe_(0.5)Mn_(0.5))O₂ was obtained in the same manner asin Example 1, except that the heat treatment step in Example 1 wasomitted.

Comparative Example 2

A positive-electrode active material represented by the compositionformula Na_(0.3)(Fe_(0.5)Mn_(0.5))O₂ was obtained in the same manner asin Example 1, except that the mixing step involved 0.15 mol of sodiumcarbonate.

Comparative Example 3

A positive-electrode active material represented by the compositionformula Na(Fe_(0.5)Mn_(0.5))O₂ was obtained in the same manner as inExample 1, except that the mixing step involved 0.50 mol of sodiumcarbonate.

Example 3 Precipitate Formation Step

Iron (II) sulfate and manganese sulfate were dissolved in pure watersuch that the ratio of the amounts thereof by mole would be Fe:Mn=2:8.Thus, a reaction field consisting of an aqueous transition metal sulfatesalt solution was prepared. The liquid temperature of the reaction fieldwas adjusted to 50° C. While performing stirring, a prescribed amount ofan aqueous sodium hydroxide solution was dropped to the reaction field,thereby obtaining a precipitate of a transition metal compositehydroxide represented by the composition formula(Fe_(0.2)Mn_(0.8))(OH)₂.

Heat Treatment Step and Subsequent Steps

The precipitate was treated in the same manner as in Example 1 to give apositive-electrode active material represented by the compositionformula Na_(0.2)(Fe_(0.2)Mn_(0.8))O₂.

Comparative Example 4 Precipitate Formation Step

Iron (II) sulfate and manganese sulfate were dissolved in pure watersuch that the ratio of the amounts thereof (by mole) would be Fe:Mn=8:2.Thus, a reaction field consisting of an aqueous transition metal sulfatesalt solution was prepared. The liquid temperature of the reaction fieldwas adjusted to 50° C. While performing stirring, a prescribed amount ofan aqueous sodium hydroxide solution was dropped to the reaction field,thereby obtaining a precipitate of a transition metal compositehydroxide represented by the composition formula(Fe_(0.8)Mn_(0.2))(OH)₂.

[Heat Treatment Step and Subsequent Steps]

The precipitate was treated in the same manner as in Example 1 to give apositive-electrode active material represented by the compositionformula Na_(0.7)(Fe_(0.8)Mn_(0.2))O₂.

Comparative Example 5 Precipitate Formation Step

Manganese sulfate was dissolved in pure water to form a reaction fieldconsisting of an aqueous manganese sulfate solution. The liquidtemperature of the reaction field was adjusted to 50° C. Whileperforming stirring, a prescribed amount of an aqueous sodium hydroxidesolution was dropped to the reaction field, thereby obtaining aprecipitate of manganese hydroxide represented by the compositionformula Mn(OH)₂.

Heat Treatment Step and Subsequent Steps

The precipitate was treated in the same manner as in Example 1 to give apositive-electrode active material represented by the compositionformula Na_(0.7)MnO₂.

Example 4 Precipitate Formation Step

Iron (II) sulfate and cobalt sulfate were dissolved in pure water suchthat the ratio of the amounts thereof by mole would be Fe:Co=1:1. Thus,a reaction field consisting of an aqueous transition metal sulfate saltsolution was prepared. The liquid temperature of the reaction field wasadjusted to 50° C. While performing stirring, a prescribed amount of anaqueous sodium hydroxide solution was dropped to the reaction field,thereby obtaining a precipitate of a transition metal compositehydroxide represented by the composition formula(Fe_(0.5)Co_(0.5))(OH)₂.

Heat Treatment Step and Subsequent Steps

The precipitate was treated in the same manner as in Example 1 to give apositive-electrode active material represented by the compositionformula Na_(0.7)(Fe_(0.5)Co_(0.5))O₂.

Example 5 Precipitate Formation Step

Iron (II) sulfate and nickel sulfate were dissolved in pure water suchthat the ratio of the amounts thereof (by mole) would be Fe:Ni=1:1.Thus, a reaction field consisting of an aqueous transition metal sulfatesalt solution was prepared. The liquid temperature of the reaction fieldwas adjusted to 50° C. While performing stirring, a prescribed amount ofan aqueous sodium hydroxide solution was dropped to the reaction field,thereby obtaining a precipitate of a transition metal compositehydroxide represented by the composition formula(Fe_(0.5)Ni_(0.5))(OH)₂.

Heat Treatment Step and Subsequent Steps

The precipitate was treated in the same manner as in Example 1 to give apositive-electrode active material represented by the compositionformula Na_(0.7)(Fe_(0.5)Ni_(0.5))O₂.

Next, the second embodiment will be described based on some examples.

Example 6 Precipitate Formation Step

Iron (II) sulfate and manganese sulfate were dissolved in pure watersuch that the ratio of the amounts thereof by mole would be Fe:Mn=1:1.Thus, a reaction field consisting of an aqueous transition metal sulfatesalt solution was prepared. The liquid temperature of the reaction fieldwas adjusted to 50° C. While performing stirring, a prescribed amount ofan aqueous sodium hydroxide solution was dropped and simultaneously aprescribed amount of CO₂ gas was blown to the reaction field, therebyobtaining a precipitate of a transition metal composite carbonate saltrepresented by the composition formula (Fe_(0.5)Mn_(0.5))CO₃. Theprecipitate was separated from the reaction field and was dried into apowder.

Mixing Step

A raw material mixture was obtained by mixing 0.35 mol of sodiumcarbonate and 1.0 mol of the precipitate of the transition metalcomposite carbonate salt.

Calcination Step

The raw material mixture was calcined in air at 900° C. to give apositive-electrode active material represented by the compositionformula Na_(0.7)(Fe_(0.5)Mn_(0.5))O₂.

Comparative Example 6

A positive-electrode active material represented by the compositionformula Na_(0.3)(Fe_(0.5)Mn_(0.5))O₂ was obtained in the same manner asin Example 6, except that the mixing step involved 0.15 mol of sodiumcarbonate.

Comparative Example 7

A positive-electrode active material represented by the compositionformula Na(Fe_(0.5)Mn_(0.5))O₂ was obtained in the same manner as inExample 6, except that the mixing step involved 0.50 mol of sodiumcarbonate.

Example 7 Precipitate Formation Step

Iron (II) sulfate and manganese sulfate were dissolved in pure watersuch that the ratio of the amounts thereof by mole would be Fe:Mn=2:8.Thus, a reaction field consisting of an aqueous transition metal sulfatesalt solution was prepared. The liquid temperature of the reaction fieldwas adjusted to 50° C. While performing stirring, a prescribed amount ofan aqueous sodium hydroxide solution was dropped and simultaneously aprescribed amount of CO₂ gas was blown to the reaction field, therebyobtaining a precipitate of a transition metal composite carbonate saltrepresented by the composition formula (Fe_(0.2)Mn_(0.8))CO₃. Theprecipitate was separated from the reaction field and was dried into apowder.

Mixing Step and Subsequent Step

The precipitate was treated in the same manner as in Example 6 to give apositive-electrode active material represented by the compositionformula Na_(0.7)(Fe_(0.2)Mn_(0.8))O₂.

Comparative Example 8 Precipitate Formation Step

Iron (II) sulfate and manganese sulfate were dissolved in pure watersuch that the ratio of the amounts thereof by mole would be Fe:Mn=8:2.Thus, a reaction field consisting of an aqueous transition metal sulfatesalt solution was prepared. The liquid temperature of the reaction fieldwas adjusted to 50° C. While performing stirring, a prescribed amount ofan aqueous sodium hydroxide solution was dropped and simultaneously aprescribed amount of CO₂ gas was blown to the reaction field, therebyobtaining a precipitate of a transition metal composite carbonate saltrepresented by the composition formula (Fe_(0.8)Mn_(0.2))CO₃. Theprecipitate was separated from the reaction field and was dried into apowder.

Mixing Step and Subsequent Step

The precipitate was treated in the same manner as in Example 6 to give apositive-electrode active material represented by the compositionformula Na_(0.7)(Fe_(0.8)Mn_(0.2))O₂.

Comparative Example 9 Precipitate Formation Step

Manganese sulfate was dissolved in pure water to form a reaction fieldconsisting of an aqueous manganese sulfate solution. The liquidtemperature of the reaction field was adjusted to 50° C. Whileperforming stirring, a prescribed amount of an aqueous sodium hydroxidesolution was dropped and simultaneously a prescribed amount of CO₂ gaswas blown to the reaction field, thereby obtaining a precipitate ofmanganese carbonate represented by the composition formula MnCO₃. Theprecipitate was separated from the reaction field and was dried into apowder.

Mixing Step and Subsequent Step

The precipitate was treated in the same manner as in Example 6 to give apositive-electrode active material represented by the compositionformula Na_(0.7)MnO₂.

Example 8 Precipitate Formation Step

Iron (II) sulfate and cobalt sulfate were dissolved in pure water suchthat the ratio of the amounts thereof by mole would be Fe:Co=1:1. Thus,a reaction field consisting of an aqueous transition metal sulfate saltsolution was prepared. The liquid temperature of the reaction field wasadjusted to 50° C. While performing stirring, a prescribed amount of anaqueous sodium hydroxide solution was dropped and simultaneously aprescribed amount of CO₂ gas was blown to the reaction field, therebyobtaining a precipitate of a transition metal composite carbonate saltrepresented by the composition formula (Fe_(0.5)Co_(0.5))CO₃. Theprecipitate was separated from the reaction field and was dried into apowder.

[Mixing Step and Subsequent Step]

The precipitate was treated in the same manner as in Example 6 to give apositive-electrode active material represented by the compositionformula Na_(0.7)(Fe_(0.5)Co_(0.5))O₂.

Example 9 Precipitate Formation Step

Iron (II) sulfate and nickel sulfate were dissolved in pure water suchthat the ratio of the amounts thereof by mole would be Fe:Ni=1:1. Thus,a reaction field consisting of an aqueous transition metal sulfate saltsolution was prepared. The liquid temperature of the reaction field wasadjusted to 50° C. While performing stirring, a prescribed amount of anaqueous sodium hydroxide solution was dropped and simultaneously aprescribed amount of CO₂ gas was blown to the reaction field, therebyobtaining a precipitate of a transition metal composite carbonate saltrepresented by the composition formula (Fe_(0.5)Ni_(0.5))CO₃. Theprecipitate was separated from the reaction field and was dried into apowder.

Mixing Step and Subsequent Step

The precipitate was treated in the same manner as in Example 6 to give apositive-electrode active material represented by the compositionformula Na_(0.7)(Fe_(0.5)Ni_(0.5))O₂.

Example 10 Precipitate Formation Step

A precipitate of a transition metal composite carbonate salt representedby the composition formula (Fe_(0.5)Mn_(0.5))CO₃ was obtained in thesame manner as in Example 6. The precipitate was separated from thereaction field and was dried into a powder.

Heat Treatment Step

The transition metal composite carbonate salt was heat treated in air at900° C. for 12 hours to give a transition metal composite oxiderepresented by the composition formula (Fe_(0.5)Mn_(0.5))₂O₃.

Mixing Step

After the heat treatment, 0.35 mol of sodium carbonate and 1.0 mol ofthe transition metal composite oxide were mixed with each other to givea raw material mixture.

Calcination Step

The raw material mixture was calcined in air at 900° C. to give apositive-electrode active material represented by the compositionformula Na_(0.7)(Fe_(0.5)Mn_(0.5))O₂.

Example 11 Precipitate Formation Step

Iron (II) sulfate and manganese sulfate were dissolved in pure watersuch that the ratio of the amounts thereof (by mole) would be Fe:Mn=1:1.Thus, a reaction field consisting of an aqueous transition metal sulfatesalt solution was prepared. The liquid temperature of the reaction fieldwas adjusted to 50° C. While performing stirring, prescribed amounts ofan aqueous sodium hydroxide solution and an aqueous oxalic acid solutionwere dropped to the reaction field, thereby obtaining a precipitate of atransition metal composite oxalate salt represented by the compositionformula (Fe_(0.5)Mn_(0.5))(COO)₂. The precipitate was separated from thereaction field and was dried into a powder.

Mixing Step

After a drying treatment, 0.35 mol of sodium carbonate and 1.0 mol ofthe precipitate of the transition metal composite oxalate salt weremixed with each other to give a raw material mixture.

Calcination Step

The raw material mixture was calcined in air at 900° C. to give apositive-electrode active material represented by the compositionformula Na_(0.7)(Fe_(0.5)Mn_(0.5))O₂.

Finally, for comparison, a conventional process will be described below.

Comparative Example 10

Powders of iron (II) oxide, manganese (III) oxide and sodium carbonatewere mixed together such that the ratio of the amounts thereof (by mole)would be Na:Fe:Mn=0.7:0.5:0.5. The resultant raw material mixture wascalcined in air at 900° C. for 12 hours to give a positive-electrodeactive material represented by the composition formulaNa_(0.2)(Fe_(0.5)Mn_(0.5))O₂.

Evaluations of Positive-Electrode Active Materials

The positive-electrode active materials of Examples 1 to 11 andComparative Examples 1 to 10 were subjected to the followingevaluations.

1. XRD Measurement

With a powder X-ray diffractometer, XRD spectra of thepositive-electrode active materials were obtained. As the X-ray, CuK α1radiation (λ=1.540562 Å) was used. The measurement was performed underconditions of a tube current of 40 mA and a tube voltage of 40 kV.

2. Evaluation of Moisture Proofness

The positive-electrode active material weighing 20 g was placed onto atray and was allowed to stand in a thermostatic chamber in which the dewpoint was 20° C. After 24 hours and 72 hours, the positive-electrodeactive material was heated at 200° C. The weight change between beforeand after the heating was calculated to obtain the moisture absorptionrate of the positive-electrode active material.

Evaluation of Battery Characteristics

Sample batteries were manufactured using the positive-electrode activematerials of Examples 1 to 11 and Comparative Examples 1 to 10, andbattery characteristics were evaluated.

1. Fabrication of Sample Batteries

1-1. Preparation of Positive-Electrodes

In NMP (N-methyl-2-pyrrolidone), 90 parts by weight of thepositive-electrode composition, 5.0 parts by weight of acetylene blackand 5.0 parts by weight of PVDF (polyvinylidene fluoride) were dispersedto give a positive-electrode slurry. The positive-electrode slurry wasapplied to an aluminum foil as a collector, dried, and compressionformed with a roll press machine. The positive-electrodes were cut to aprescribed size.

1-2. Preparation of Negative Electrodes

In NMP (N-methyl-2-pyrrolidone), 95 parts by weight of hard carbon and5.0 parts by weight of PVDF (polyvinylidene fluoride) were dispersed togive a negative electrode slurry. The negative electrode slurry wasapplied to an aluminum foil as a collector, dried, and compressionformed with a roll press machine. The negative electrodes were cut to aprescribed size.

1-3. Preparation of Non-Aqueous Electrolytic Solution

EC (ethylene carbonate) and DEC (diethyl carbonate) were mixed with eachother in a volume ratio of 1:1 to give a mixed solvent. Sodiumhexafluorophosphate (NaPF₆) was dissolved in the mixed solvent with aconcentration of 1 mol/l. Thus, a non-aqueous electrolytic solution wasobtained.

1-4. Assembling of Sample Batteries

Lead electrodes were attached to the respective collectors of thepositive-electrode and the negative electrode, and the electrodes weredried in vacuum at 120° C. Next, a porous polyethylene separator wasprovided between the positive-electrode and the negative electrode, andthe unit was placed into a laminate pack in the form of a bag. Thepackage was dried in vacuum at 60° C. to remove water that had beenadsorbed onto the components. After the vacuum drying, the non-aqueouselectrolytic solution was poured into the laminate pack, and the packwas sealed. In this manner, laminate-type samples of non-aqueouselectrolyte secondary batteries were obtained.

2. Charge and Discharge Characteristics

A weak current was applied to the sample battery to perform aging, andthereby the positive-electrode and the negative electrode were allowedto sufficiently conform to the electrolyte. After the aging, the batterywas charged by constant current-constant voltage charging with a fullcharge voltage of 4.4 V and a charging rate of 0.2 C (1 C: a currentdensity at which a fully charged battery is fully discharged in 1 hour)(charging finish conditions: 0.008 C). The capacity obtained was thecharge capacity. After the full charging, the battery was discharged ata constant current with a discharging voltage of 1.7 V and a dischargingrate of 0.2 C. The capacity obtained was the discharge capacity. Theratio of the discharge capacity to the charge capacity was calculated todetermine the charging-discharging efficiency.

3. Cycle Characteristics

The constant current-constant voltage charging and the constant currentdischarging described in the charge and discharge characteristics wererepeated fifty times, and changes in discharge capacity were studied.The ratio of the discharge capacity in the n-th discharging to thedischarge capacity in the first discharging was obtained as the capacityretention Ps (n) after n cycles.

For Examples 1 to 11 and Comparative Examples 1 to 10, the productionconditions, the properties of the positive-electrode active materials,and the battery characteristics for Examples in which M was manganeseare described in Tables 1, 2 and 3, respectively, and the productionconditions, the properties of the positive-electrode active materials,and the battery characteristics for Examples in which M was cobalt ornickel are described in Tables 4, 5 and 6, respectively. For Examples 1and 6 and Comparative Examples 1 and 10, the XRD spectra of thepositive-electrode active materials are shown in FIG. 1, the moistureabsorption properties of the positive-electrode active materials areshown in FIG. 2, and the cycle characteristics of the secondarybatteries are shown in FIG. 3. In FIG. 1, peaks assigned to the (016)plane are seen in the region where 2θ is about 61.5°, and peaks assignedto the (110) plane are found in the region where 2θ is about 63.7°. TheXRD spectra in Examples 2 to 5 and 7 to 11 and in Comparative Examples 5and 9 were similar to those of Examples 1 and 6.

TABLE 1 Heat Transition treatment metal temperature/ x y M ions aq.Precipitate °C. Ex. 1 0.7 0.5 Mn Sulfate Hydroxide 900 Ex. 2 salt aq.650 Comp. — Ex. 1 Comp. 0.3 900 Ex. 2 Comp. 1 Ex. 3 Ex. 3 0.7 0.8 Comp.0.2 Ex. 4 Comp. 1 Ex. 5 Ex. 6 0.7 0.5 Mn Sulfate Carbonate — Comp. 0.3salt aq. salt Ex. 6 Comp. 1 Ex. 7 Ex. 7 0.7 0.8 Comp. 0.2 Ex. 8 Comp. 1Ex. 9 Ex. 10 0.5 Mn 900 Ex. 11 0.5 Oxalate — salt Comp. 0.7 0.5 Mn — — —Ex. 10

TABLE 2 (016) (110) (110)/(016) Moisture Crystal integral Integralintensity SO₄ ²⁻/ absorption phase width/° width/° ratio wt % rate*/%Ex. 1 P6₃/mmc 0.31 0.17 0.55 <0.03 1.5 Ex. 2 0.43 0.23 0.53  0.10 3.2Comp. 1.36 0.30 0.22  0.72 8.0 Ex. 1 Comp. Unidenti- 0.45 0.22 0.49<0.03 1.3 Ex. 2 fiable Comp. R-3m 0.36 0.22 0.61 <0.03 1.5 Ex. 3 Ex. 3P6₃/mmc 0.45 0.21 0.47 <0.03 4.5 Comp. R-3m•Pna21 0.23 0.19 0.83 <0.031.8 Ex. 4 Comp. P6₃/mmc 0.52 0.22 0.42 <0.03 15.0  Ex. 5 Ex. 6 P6₃/mmc0.43 0.16 0.37 <0.03 1.3 Comp. Unidenti- 0.48 0.22 0.46 <0.03 1.4 Ex. 6fiable Comp. R-3m 0.42 0.20 0.48 <0.03 1.5 Ex. 7 Ex. 7 P6₃/mmc 0.45 0.200.44 <0.03 4.5 Comp. R-3m•Pna21 0.22 0.19 0.86 <0.03 2.0 Ex. 8 Comp.0.52 0.21 0.40 <0.03 10.0  Ex. 9 Ex. 10 P6₃/mmc 0.32 0.18 0.56 <0.03 1.5Ex. 11 0.32 0.19 0.59 n.a. 1.8 Comp. P6₃/ 0.29 0.45 1.55 <0.03 1.0 Ex.10 mmc•R-3m *After 72 hr n.a.: not available

TABLE 3 Charge Discharge Charging- capacity/ capacity/ dischargingmAhg⁻¹ mAhg⁻¹ efficiency/% Ps(50)/% Ex. 1 122 97 79 92 Ex. 2 123 98 7988 Comp. Ex. 1 124 98 79 76 Comp. Ex. 2 55 42 76 85 Comp. Ex. 3 133 10579 75 Ex. 3 125 96 77 90 Comp. Ex. 4 106 78 73 72 Comp. Ex. 5 123 94 7685 Ex. 6 124 97 78 96 Comp. Ex. 6 54 43 80 85 Comp. Ex. 7 134 106 79 73Ex. 7 125 95 76 88 Comp. Ex. 8 105 78 74 71 Comp. Ex. 9 124 94 76 84 Ex.10 124 97 78 93 Ex. 11 124 98 79 92 Comp. Ex. 10 123 92 75 92

From Tables 1 and 2 and FIG. 1, the following is reached.

The peaks obtained in Examples 1 and 6 have a higher intensity and anarrower integral width than the peaks of Comparative Example 1,indicating that the P6₃/mmc structure achieved as high crystallinity asthat obtained in Comparative Example 10. This result is probablyascribed to the removal of counter ions (sulfate ions in this case) bythe heat treatment. On the other hand, the spectrum of ComparativeExample 10 shows peaks indicating the presence of undesired R-3m phase(the three peaks enclosed with a broken line in FIG. 1). This result isprobably because the elements were not uniformly mixed together in themixing of the raw materials.

From Tables 1 and 2 and FIG. 2, the following is reached.

Omitting the heat treatment step in the first embodiment allows a largeamount of sulfate ions to remain and hence results in high moistureabsorption rate as illustrated in Comparative Example 1. The amount ofresidual sulfate ions is sufficiently decreased and the moistureabsorption rate is lowered by performing the heat treatment asdemonstrated in Example 1 or by obtaining the positive-electrode activematerial in accordance with the second embodiment as illustrated inExample 6.

From Tables 1 to 3 and FIG. 3, the following is obtained.

The non-aqueous electrolyte secondary batteries using thepositive-electrode active materials of Examples 1 and 6 outperformed thebattery based on Comparative Example 1 in terms of cyclecharacteristics. This result is probably attributed to the difference incrystallinity between Examples 1 and 6 and Comparative Example 1. On theother hand, the non-aqueous electrolyte secondary batteries using thepositive-electrode active materials of Examples 1 and 6 achieved highercharging-discharging efficiency than the battery based on ComparativeExample 10. This result is probably ascribed to the presence or absenceof subphases other than the P6₃/mmc structure.

From Tables 1 to 3, the following is reached.

Values of x falling far outside the range of 0.4≦x≦0.7 cause a failurefor the positive-electrode active material to have a P6₃/mmc singlephase structure and result in a decrease in charge-discharge capacity ora deterioration in cycle characteristics as demonstrated in ComparativeExamples 2, 3, 6 and 7. Crystallinity is lowered or subphases are causedto occur if the value of y is outside the range of 0.25≦y<1.0 asillustrated in Comparative Examples 4, 5, 8 and 9.

TABLE 4 Heat Transition treatment metal ions temperature/ x y M aq.Precipitate °C. Ex. 4 0.7 0.5 Co Sulfate Hydroxide 900 salt aq. Ex. 50.7 0.5 Ni Sulfate Hydroxide 900 salt aq. Ex. 8 0.7 0.5 Co SulfateCarbonate — salt aq. salt Ex. 9 0.7 0.5 Ni Sulfate Carbonate — salt aq.salt

TABLE 5 (016) (110) (110)/(016) Moisture Crystal integral Integralintensity SO₄ ²⁻/ absorption phase width/° width/° ratio wt % rate*/%Ex. 4 P6₃/mmc 0.35 0.22 0.63 n.a. 1.3 Ex. 5 P6₃/mmc 0.40 0.25 0.63 n.a.1.2 Ex. 8 P6₃/mmc 0.33 0.24 0.73 n.a. 1.5 Ex. 9 P6₃/mmc 0.35 0.21 0.60n.a. 1.1 *After 72 hr n.a.: not available

TABLE 6 Charge Discharge Charging- capacity/ capacity/ dischargingmAhg⁻¹ mAhg⁻¹ efficiency/% Ps(50)/% Ex. 4 132  98 74 92 Ex. 5 140 101 7292 Ex. 8 133  99 74 89 Ex. 9 138 103 75 93

From Tables 4 to 6, the following is obtained.

Even when M in the formula was replaced manganese with cobalt or nickel,the obtained peaks had a high intensity and a narrow integral width,showing high crystallinity of the P6₃/mmc structure. Further, thenon-aqueous electrolyte secondary batteries obtained in such casesexhibited high charging-discharging efficiency and good cyclecharacteristics.

As demonstrated above, the positive-electrode active materials of theinvention have good charge and discharge characteristics and good cyclecharacteristics, and are substantially free from residual counter ions.Thus, the inventive positive-electrode active materials may be suitablyused in non-aqueous sodium secondary batteries. Further, the inventivemethods for the producing of positive-electrode active materials canproduce positive-electrode active materials which are satisfactory bothin charge and discharge characteristics and cycle characteristics and inother properties.

The foregoing description of the exemplary embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in the art. Theexemplary embodiments were chosen and described in order to best explainthe principles of the invention and its practical applications, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with the various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

All publications, patent applications, and technical standards mentionedin this specification are herein incorporated by reference to the sameextent as if each individual publication, patent application, ortechnical standard was specifically and individually indicated to beincorporated by reference.

What is claimed is:
 1. A positive-electrode active material fornon-aqueous secondary battery comprising a sodium transition metalcomposite oxide represented by Formula:Na_(x)Fe_(1-y)M_(y)O₂, wherein 0.4≦x≦0.7, 0.25≦y<1.0, and M is at leastone element selected from the group consisting of manganese, cobalt andnickel, the sodium transition metal composite oxide having a crystalstructure substantially composed of P6₃/mmc alone.
 2. Thepositive-electrode active material according to claim 1, wherein theratio of a peak intensity assigned to the (110) plane to a peakintensity assigned to the (016) plane is 0.30 to 2.00 according topowder X-ray diffractometry.
 3. The positive-electrode active materialaccording to claim 1, wherein the integral width of a peak assigned tothe (016) plane is 0.10° to 1.00° according to powder X-raydiffractometry.
 4. The positive-electrode active material according toclaim 1, wherein the integral width of a peak assigned to the (110)plane is 0.10° to 0.30° according to powder X-ray diffractometry.
 5. Thepositive-electrode active material according to claim 1, wherein M ismanganese.
 6. A method for producing a positive-electrode activematerial for non-aqueous secondary battery comprising a sodiumtransition metal composite oxide represented by Formula:Na_(x)Fe_(1-y)M_(y)O₂, wherein 0.4≦x≦0.7, 0.25≦y<1.0, and M is at leastone element selected from the group consisting of manganese, cobalt andnickel, the method comprising: a precipitate formation step of obtaininga precipitate of a transition metal composite compound from a transitionmetal ion-containing aqueous solution, a heat treatment step of heattreating the precipitate from the precipitate formation step to obtain atransition metal composite oxide precursor, a mixing step of mixing theprecursor from the heat treatment step with at least a sodium compoundto obtain a raw material mixture, and a calcination step of calciningthe raw material mixture from the mixing step to obtain a calcinedproduct.
 7. A method for producing a positive-electrode active materialfor non-aqueous secondary battery comprising a sodium transition metalcomposite oxide represented by Formula:Na_(x)Fe_(1-y)M_(y)O₂, wherein 0.4≦x≦0.7, 0.25≦y<1.0, and M is at leastone element selected from the group consisting of manganese, cobalt andnickel, the method comprising: a precipitate formation step of obtaininga precipitate from a transition metal ion-containing aqueous solution,the precipitate containing a transition metal composite compound otherthan a hydroxide as a main component, a mixing step of mixing theprecipitate from the precipitate formation step with at least a sodiumcompound to obtain a raw material mixture, and a calcination step ofcalcining the raw material mixture from the mixing step to obtain acalcined product.
 8. The method according to claim 7, wherein the maincomponent of the precipitate is a transition metal composite carbonatesalt.
 9. The method according to claim 7, wherein the method furthercomprises a heat treatment step of heat treating the precipitate fromthe precipitate formation step to obtain a transition metal compositeoxide, and the mixing step is a step of mixing the transition metalcomposite oxide with at least a sodium compound to obtain a raw materialmixture.
 10. The method according to claim 6, wherein the heat treatmenttemperature in the heat treatment step is 600° C. to 1000° C.
 11. Themethod according to claim 9, wherein the heat treatment temperature inthe heat treatment step is 600° C. to 1000° C.
 12. The method accordingto claim 6, wherein the transition metal ion-containing aqueous solutionin the precipitate formation step is an aqueous transition metal sulfatesalt solution.
 13. The method according to claim 7, wherein thetransition metal ion-containing aqueous solution in the precipitateformation step is an aqueous transition metal sulfate salt solution. 14.A composition for a positive-electrode for non-aqueous secondary batterycomprising the positive-electrode active material according to claim 1and a binder.
 15. A positive-electrode for non-aqueous secondary batterycomprising a positive-electrode collector and a positive-electrodeactive material layer formed on the positive-electrode collector,wherein the positive-electrode active material layer comprising thepositive-electrode active material according to claim
 1. 16. Anon-aqueous secondary battery comprising a positive-electrode accordingto claim 15, a negative-electrode and non-aqueous electrolytic solution.